A variety of techniques have been developed for producing thin superconductor layers on a substrate. In one of these techniques, the material is constituted as a target or cathode material and the target is subjected to cathodic sputtering, thereby transferring particles of the material onto a substrate disposed in the immediate region of the coating source. The substrate is usually heated.
In an apparatus for this purpose, a vacuum chamber can be provided with the sputtering source and immediately in the vicinity thereof the substrate is disposed so that there is a direct transfer of the sputtered material from the cathode to the substrate.
The class of materials with which the invention is applicable, namely those which contain oxygen, nitrogen or other reactive gases, include high-temperature superconductors, including the YBa.sub.2 Cu.sub.3 O.sub.7-x, BiSrCaCuO or TlBaCaCuO types, where x is a fraction less than 1.
These compositions have a rather complex structure, referred to as a Perovskite structure and oxygen which is incorporated in situ therein.
A number of deposition techniques have been used to produce thin films of these materials, e.g. CVD, evaporation laser deposition and sputtering method. The preparation of uniform high quality films especially over large areas has been proven difficult due to certain limitations characteristic of each deposition technique. An important advantage of plasma sputtering over e.g. evaporation methods is among others given by the fact that this technique stoichiometric material as target material can be ablated in a very reproducible way at generally high reactive gas (i.e. oxygen) processes.
Earlier systems over which the present invention can be considered to be an improvement, are described, for example in X. X. Xi, G. Linker, O. Mayer, E. Nold, B. Obst, F. Ratzel, R. Smithey, B. Strehlau, F. Weschenfelder, and J. Geerk, Z. Phys. B 74 (1989), 13 or C. B. Eom, J. Z. Sun, K. Yamamoto, A. F. Marshall, K. E. Luther, T. H. Gaballe and S. S. Laderman, Appl. Phys. Lett. 55 (1989), P. 595. These techniques are referred to as "off axis" high pressure cathodic sputtering since the substrate is located with a spacing from the axis of the cathodic target.
A so-called back-sputter effect is described in L. R. Gilbert, R. Messier and S. V. Krishnaswamy, J. Vac. Sci. Technol. 17, (1980), according to which a coating deposited by conventional cathodic sputtering is removed partially by ions from the plasma.
This problem characterizes even off-axis high pressure cathodic sputtering as previously mentioned as well as other cathodic sputtering methods. In all of the earlier sputtering methods, moreover, undesirable and disadvantageous charge accumulation effects are observed.